Add CI tests

[WebFreak001]

• migrate travis unittests to github actions as it's no longer being used
• add integration test / simulate a full basic debugging routine

[GitMensch]

The first part was done in #354 by @brownts. 👍.

[brownts]

I've reworked the test harness so that it's no longer dependent on having VSCode run it. That should be a much nicer workflow, since you don't have to launch another instance of VSCode to run the tests. This just runs Mocha directly instead of through VSCode. There was really no reason to run it through VSCode as the extension doesn't dependent on it. In addition, I've also added functionality to collect code coverage and report it via CodeCov. Lastly, I've added badges in the README to show the CI status as well as the coverage percentage, along with links to the CodeCov detailed information.

The tests can be run from the "npm:test" in tasks.json if you like (or running npm test from the command line), but it is much nicer if you actually install and use the "Mocha Test Explorer" extension. That adds nice integration and provides a "Testing" tab in the VSCode Activity Bar. You can run/debug/etc. tests from there with ease.

[GitMensch]

Thank you this sounds very usefull. The next thing would be to increase the test coverage...